Dot printing machines generally have one or more printing heads which can be moved along a recording medium, usually formed by a strip of paper. Each head has a group of recording styli or needles which are selectively actuated at predetermined times, in order to print a quantity of individual dots which form a character on the paper.
In certain dot printing machines, the recording styli in each head are arranged in a rectangular matrix including 5.times.7=35 styli. A character is printed by actuating simultaneously those styli which print the dots making up the character image. In other dot printers, each head has only seven styli aligned along the paper in a row perpendicular to a print line. With the latter arrangement, a character is printed by stepping the printing head and energizing the styli required to form the dots for the appropriate part of the character between two successive steps. Thus, the whole character may be printed by the printing head in five steps.
The movement of the recording styli in dot printing machines is controlled by actuating devices associated with respective ones of the styli. For example, each actuating device may be of the type described and illustrated in French Pat. No. 2,218,746 corresponding to U.S. Pat. No. 3,971,311, wherein a flat electro dynamic coil, secured to a plate mounted to pivot about an axis parallel to the print line, is subjected to the effect of a magnetic field perpendicular to its plane. The plate is shaped to form a recording stylus which is moved in the direction of the paper when the coil is energized by a pulse of electric current.
Because the dots forming a character are in close proximity to each other and there is a short distance between the recording styli of one head, it has been found necessary to arrange the actuating devices of one head in such a way that the recording styli are aligned in a direction parallel to one of the diagonals of a printing matrix. This arrangement avoids the mechanical interaction which is likely to occur between two adjoining actuating devices. In this prior art device, the actuating devices are mounted to pivot about the same pivot axis, and the radii of the arcs they describe differ.